A TSO-DSO interactive mechanism is proposed to realize coordinated voltage support of transmission network by reactive power provision, activating flexibility of active distribution networks, such as distributed generations, flexible loads, reactive power compensators, etc. The mechanism consists of four stages: i) TSO sends reactive power request to DSOs; ii) Each DSO forms its reactive power quantity-price curve and sends to TSO; iii) TSO does a global optimization to achieve a voltage support scheme, incorporating its own reactive power provision devices and reactive power quantity-price curves; iv) Each DSO re-optimizes in its local area to reach the voltage support requirement. Based on this mechanism, the specific implementation approaches of the four stages are discussed, and a four-stage distributed algorithm is proposed. The case analysis demonstrates that the proposed mechanism and algorithm can effectively and accurately realize voltage support. Also, the TSO is able to achieve a more cost-effective scheme compared with independent voltage support while the active elements managed by DSOs achieve profits from participating in the coordinated voltage support. Specifically, in the test case presented in this paper, the coordinated scheme can save more than 50% costs and 25% reactive power requirements compared with the traditional one.